The present invention relates to a high pressure discharge lamp and a lamp unit. In particular, the present invention relates to a high pressure discharge lamp used for general illumination, projectors in combination with a reflecting mirror, headlights of automobiles or the like.
In recent years, an image projecting apparatus such as a liquid crystal projector and a DMD projector is commonly used as a system for realizing large-scale video images, and in general, a high pressure discharge lamp having a high intensity is commonly used for such an image projecting apparatus. FIG. 40 is a schematic view showing the structure of a conventional high pressure discharge lamp 1000. The lamp 1000 shown in FIG. 40 is a so-called superhigh pressure mercury lamp is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2-148561.
The lamp 1000 includes a luminous bulb (arc tube) 101 made of quartz glass and a pair of sealing portions (seal portions) 102 extending from both ends of the luminous bulb 101. A luminous material (mercury) 106 is enclosed inside (in a discharge space) of the luminous bulb 101, and a pair of tungsten electrodes (W electrodes) 103 made of tungsten are opposed with a predetermined distance. A molybdenum foil (Mo foil) 104 in the sealing portion 102 is welded to one end of the W electrode 103, and the W electrode 103 and the Mo foil 104 are electrically connected. An external lead (Mo rod) 105 made of molybdenum is electrically connected to one end of the Mo foil 104. Argon (Ar) and a small amount of halogen, in addition to the mercury 106, are enclosed in the luminous bulb 101.
The operational principle of the lamp 1000 will be described below. When a start voltage is applied between the W electrodes 103 via the external leads 105 and the Mo foils 104, discharge of argon (Ar) occurs, and this discharge increases the temperature in the discharge space of the luminous bulb 101, and then the mercury 106 is heated and evaporated. Therefore, mercury atoms are exited in the central portion of an arc between the W electrodes 103 and thus light is emitted. The higher the mercury vapor pressure of the lamp 1000 is, the more light is radiated, so that the higher mercury vapor pressure is more suitable for the light source of an image projecting apparatus, but in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 20 MPa (150 to 200 atm).
The conventional lamp 1000 has a strength against a pressure of about 20 MPa. In order to further improve the lamp characteristics, research and development are conducted to further enhance the strength against pressure (e.g., see Japanese Laid-Open Patent Publication No.2001-23570). This is because there is a demand for a higher output and power lamp to realize a higher performance image projecting apparatus, and thus there is a demand for a lamp having a high strength against pressure in order to meet this demand.
Further describing this point, in the case of a high output and power lamp, in order to suppress evaporation of the electrodes from being facilitated by an increase of current, it is necessary to enclose a higher amount of mercury than usual so as to increase the lamp voltage. If the amount of mercury enclosed is insufficient relatively to the lamp power, the lamp voltage cannot be increased to a necessary level, so that the lamp current increases. As a result, the electrodes are evaporated in a short time, and therefore a practical lamp cannot be achieved. In other words, what should be done in order to realize a high power lamp is only to increase the lamp power and to produce a short-arc type lamp whose interelectrode distance is shorter than a conventional lamp. However, in order to produce a high output and high power lamp in practice, it is necessary to improve the strength against pressure to increase the amount of mercury enclosed. Current techniques have not succeeded in realizing a high pressure discharge lamp having a very high strength against pressure (e.g., about 30 MPa or more) that can be used in practice.